1. Field of the Invention
The present invention relates to an air-fuel ratio sensor for an internal combustion engine, which is capable of detecting an air-fuel ratio of fuel mixture supplied to the engine both in the rich and lean regions as well as the stoichiometric air-fuel ratio, and more particularly to an improvement of an air-fuel ratio sensor of the type that an air-fuel ratio of the fuel mixture can be identified by simply amplifying a voltage produced by a sensing portion of the sensor. The present invention further relates to an air-fuel ratio control apparatus using the same.
2. Description of the Related Art
There has been known an air-fuel ratio sensor of the type that an air-fuel ratio of fuel mixture supplied to an internal combustion engine is detected directly from a voltage produced by a sensing portion made of solid electrolyte, oxide semiconductor and so on in response to the concentration of remaining oxygen included in exhaust gas from the engine over the wide range from the rich region to the lean one, including the stoichiometric air-fuel ratio (Japanese patent laid-open publication JP-A-62/67255 published Mar. 26, 1987).
According to the conventional technology as described above, there are provided means for detecting both a voltage induced by a sensing portion and an internal resistance thereof. The temperature dependence of the output voltage, which is accompanied by the sensing portion, is compensated by using the detected internal resistance thereof.
In the conventional sensor, however, there are problems in view of the accuracy and reliability of the measurement. Especially, the measurement accuracy of the sensor becomes worse in the lean and rich regions of an air-fuel ratio of fuel mixture, because the changing rate of a voltage induced by the sensing portion with respect to an air-fuel ratio is considerably small in those regions.